Generally, the key requirements of a tissue adhesive are:                (1) In use, the adhesive must mimic the mechanical performance of the undamaged tissue;        (2) The adhesive should provide sufficient tack for “primary” fixation with the opportunity for manipulation and re-alignment prior to setting strongly;        (3) Any exothermic process involved in the curing of the adhesive should not damage the surrounding tissue;        (4) The adhesive must not elicit any toxic response by the surrounding healthy tissue and should facilitate the re-growth of new tissue where possible;        (5) The adhesive should not liberate harmful degradation products;        (6) The adhesive should degrade, and as it does so, it should be replaced by new tissue with minimal scarring; and        (7) Any biodegradation products should not accumulate in the body but should be eliminated naturally either by excretion or incorporation into the natural biochemical cycle.        
[“Polymeric Biomaterials”, 2nd Ed., Marcel Dekker Inc., (2002) pp. 716]
It is well known in the art that diisocyanate monomers may be used to form polymeric adhesives. However, many of the diisocyanate monomers that are commercially available are small molecule diisocyanate monomers that present toxicity and sensitization hazards and that polymerize to form products having toxic degradation products, for instance, aromatic amines. As such, commercially available small molecule diisocyanate monomers are unsuitable for human use as an internal adhesive or sealant.
Metabolically acceptable polyisocyanate monomers are described in U.S. Pat. No. 4,829,099. More specifically, this reference describes an aromatic benzoyl isocyanate terminated monomer, having glycolic acid residues and polyethyleneglycol residues, in formula “I, Preferred”. This reference indicates that the resultant polymer will degrade ultimately to metabolically acceptable products, including p-aminobenzoic acid, polyethylene glycol and glycolic acid. Although the resultant polymer in principal could degrade into the aforementioned compounds, it is believed that only the glycolic acid residues would hydrolyse in vivo, resulting in a mixture of water-soluble and water insoluble fragments. The water-soluble fragments would be eliminated naturally by excretion from the body. However, the water insoluble fragments would not be eliminated naturally, resulting in the undesirable accumulation of the water insoluble fragments in the body.
Polyester-urethane-urea block copolymers prepared from commercially available small molecular diisocyanates, i.e. tolylene diisocyanate (TDI), diphenylmethane-4,4′-diisocyanate (MDI), and hexamethylene disisocyanate (HMDI), are described in U.S. Pat. No. 6,210,441. However, these copolymers would be unsuitable for use as a surgical adhesive or sealant, since the copolymers are already polymerized, i.e., already cured, and would not provide sufficient opportunity for manipulation and re-alignment. Moreover, such copolymers are not believed to mimic the mechanical performance of undamaged tissue.
Therefore, it is desirable to have a monomer based internal adhesive or sealant formulation that is capable of polymerizing in vivo to form an internal adhesive or sealant, in order to provide an opportunity for manipulation and re-alignment. Specifically, it is desirable that the adhesive or sealant formulation fills internal cavities and voids, penetrating and conforming to the interstices and pores of the tissue, prior to curing or setting.
Additionally, it is desirable to have a monomer based internal adhesive or sealant formulation that polymerizes in vivo, where the monomer, the formulation thereof, and the resultant polymer are biocompatible. The resultant polymer should also be biodegradable.
Finally, it is desirable that the degradation products of the resultant polymer be both biocompatible and water soluble, so that the degradation products are completely eliminated from the human body as waste products.